3 July 1884: Charles Dow starts counting

Charles Dow published his first stock average in the Customers' Afternoon Letter, the forerunner of The Wall Street Journal. It tracked just 11 stocks, most of them railroads, the tech giants of the day. The idea was simple: one number to tell you roughly how the market was doing. That humble average evolved into the Dow Jones Industrial Average, and the railroads that dominated it are a reminder that today's giants rarely stay giants.

The lesson: Indexes outlive their members; the companies at the top today are unlikely to be the leaders of tomorrow.